The present invention relates to a droplet generator for generating streams of ink droplets in a continuous ink jet (CIJ) printer, and a method of operating a droplet generator.
Conventionally, a CIJ printer comprises a print head containing an ink-receiving cavity and a row of nozzles that lead from the cavity through one face of the head. Ink is fed into the cavity at a high pressure (typically 2 to 3 bar) and emerges through the nozzles as a series of high velocity jets. The head is designed to be highly resonant and it is driven at a resonant frequency by one or more piezoelectric transducers. The resonance of the head modulates the pressure of the ink as it emerges from the nozzles and this causes the ink jets to break up into streams of droplets at the modulation frequency.
Selected ink droplets may be electrostatically charged, so that they can be deflected by an electric field. Deflected droplets are collected and this collected ink is conditioned and recycled for re-use. Uncharged droplets are not deflected by the electric field and thus continue in a straight line until they strike a substrate, thereby printing an image on the substrate.
CIJ printers operate at very high speeds (up to ten times faster than conventional drop-on-demand ink jet printers) but are very much more expensive than drop-on-demand printers. They are therefore used mainly where very large print volumes are required: for example, they are sometimes used to print information on packets of food or pharmaceuticals as the packets move along a production line.
Conventional CIJ printers have a number of other drawbacks, apart from their high cost. In particular, such printers are very sensitive to factors that might affect the resonance of the print head, such as component manufacturing tolerances and assembly conditions, and sources of acoustic disturbance in the print head such as cavitation or the presence of air bubbles. Any such imperfections can cause serious deterioration in print quality. Construction and maintenance of the print head is therefore difficult and expensive and very careful conditioning of the ink is required.
The above difficulties arise mainly from the need to generate a defined mode of resonance in which the pressure modulations are identical at all of the nozzles. All other modes of resonance should be suppressed. This is extremely difficult to achieve in practice since three dimensional structures like conventional CIJ print heads have very many possible modes of resonance having closely spaced resonant frequencies. It is therefore very difficult to drive the desired resonant mode without also driving various other unwanted modes.
In order to allow only the desired resonant mode to be driven, conventional CIJ heads are designed to produce very sharp resonances. The bandwidth of the possible drive frequencies for each resonant mode is thereby reduced, which makes it possible to drive the desired mode without driving nearby unwanted modes. An undesirable consequence of this is however that the head is very sensitive to minor variations in the characteristics of the ink or the presence of air bubbles, as mentioned above. The head is also very expensive to manufacture and maintain.
These difficulties also place practical limits on both the size and the operational frequency of the print head. The maximum operational frequency achievable at present with a standard 50 mm (2xe2x80x3) print head is normally about 100 kHz and the maximum practical length for a commercially manufactured print head is thought to be approximately 100 mm (4xe2x80x3).
It is therefore desirable to provide a longer print head that is capable of operating at higher frequencies. However, this would require further improved inhibition of unwanted resonance modes, when such inhibition is already difficult with print heads of conventional dimensions.
It is an object of the present invention to provide a droplet generator that is suitable for use as a print head in a CIJ printer, and a method of operating such a printer, that mitigates at least some of the afore-mentioned disadvantages.
In accordance with one aspect of the present invention, there is provided a droplet generator for generating streams of ink droplets in a continuous ink jet printer, the droplet generator including a flexible stimulator plate having a plurality of nozzles that extend through the plate from one face to the other, said nozzles being arranged in at least one substantially rectilinear line, means for supplying ink under pressure to the nozzle-bearing region of one face of the stimulator plate, and actuator means for generating bending vibrations in the stimulator plate, said actuator means including at least one electromechanical transducer that is arranged to expand and contract in a direction parallel to the plane of the stimulator plate, the or each said line of nozzles being located to coincide, during use, with a locus of substantially uniform vibrational amplitude and phase, whereby substantially identical streams of ink droplets are ejected from the nozzles.
The piezoelectric transducers are arranged to expand or contract in the plane of the stimulator plate (i.e. perpendicular to the local surface normal of the stimulator plate) when an electric field is applied thereto. This causes bending of the stimulator plate, thereby driving the vibrations of the generator in a simple and efficient manner. Preferably, a standing wave is generated that provides at least one substantially rectilinear antinode or antinodal line, that is a point or a locus of points vibrating in phase whose amplitude is a local maximum when traversed in at least one direction in that portion of the flexible membrane that is contacted by the ink. The line of nozzles is preferably positioned parallel to and either adjacent to or on an antinodal line of said standing wave.
Providing a linear array of nozzles that coincides with a locus of substantially uniform vibrational amplitude ensures that all the nozzles experience similar vibrations. The pressure modulations in the ink jets emerging from the nozzles are therefore substantially identical, resulting in identical streams of ink droplets.
Because the stimulator plate is, as far as its acoustic properties are concerned, a two dimensional structure, it has many fewer possible modes of resonance than a conventional three-dimensional droplet generator and the frequencies of those resonant modes are correspondingly much more widely spaced. It is therefore relatively easy to drive only the desired mode without driving other undesired modes. Advantageously, the vibration of the stimulator plate generates a stimulation pressure in ink directly in contact with a region of the flexible plate member that narrowly encompasses the nozzles. In this way the path length traveled through the ink by the acoustic energy is kept to a minimum, thereby also providing a decoupling of the stimulation pressure so produced from the acoustic effects of the other, more distant, boundaries of the ink. This is unlike conventional CIJ droplet generators, in which the ink itself plays an acoustically active role in the resonance of the stimulator and which are thus very sensitive to changes in conditions pertaining, such as in the acoustic characteristics of all the solid boundaries of the ink or to the accidental intrusion of a bubble of air.
This means that in the new generator a very sharp resonance is not necessary and the droplet generator is thus far less sensitive to factors such as the consistency of the ink or the presence of air bubbles, which generally have a very serious effect on the performance of a conventional droplet generator. The new droplet generator is therefore cheaper and more reliable in operation and does not require such complicated ink conditioning apparatus.
A further advantage resulting from the use of an acoustically two-dimensional stimulator plate is that the vibrating part of such a structure can have a much lower mass and acoustic impedance than in a conventional droplet generator, the acoustic impedance of the vibrating flexible plate member being comparable to that of the actuators. These circumstances mean that the amount of vibrational energy stored in the stimulator plate is smaller than that stored in conventional CIJ stimulators and that a larger amount of energy can be transferred per cycle in either direction between the actuators and the vibrating flexible plate member. This makes it possible to control the vibration of the stimulator directly by feeding appropriate drive signals to the actuators, so allowing unwanted modes to be actively suppressed. Conventional CIJ droplet generators have an acoustic impedance at their operating frequency that is much larger than the acoustic impedance of the piezoelectric actuators, making resonance control by means of an electrical drive signal supplied to the actuators much more difficult to achieve.
Finally, the new droplet stimulator plate can operate successfully at higher frequencies than conventional droplet generators (for example, at frequencies exceeding 150 kHz), with no practical upper limit on the dimension of the stimulator plate and the droplet generator in the direction of the line of nozzles.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.